Semiconductor device

ABSTRACT

A semiconductor device of an embodiment includes: a semiconductor layer made of p-type nitride semiconductor; an oxide layer formed on the semiconductor layer, the oxide layer being made of a crystalline nickel oxide, and the oxide layer having a thickness of 3 nm or less; and a metal layer formed on the oxide layer.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of, and claims the benefit ofpriority under 35 U.S.C. §120, from U.S. application Ser. No.13/035,069, filed Feb. 25, 2011, which claims the benefit of priorityunder 35 U.S.C. §119, from Japanese Patent Applications No. 2010-198632,filed Sep. 6, 2010 in Japan, the entire contents of all of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor device.

BACKGROUND

A nitride semiconductor is a wide-gap semiconductor having a wide bandgap and is used as a substrate of a laser diode (LD), a semiconductorlight emitting diode (LED), or the like by using the wide band gap. Thenitride semiconductor has firm crystal and withstands large current andhigh voltage. Consequently, application to a heterojunction bipolartransistor of high output and the like is also considered.

Challenges of a semiconductor device using such a nitride semiconductoras a substrate are decrease in operation voltage and improvement inreliability. To solve the challenges, realization of a contact electrodestructure having low resistance to a nitride semiconductor and highreliability is required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic cross sections of a semiconductor deviceof a first embodiment;

FIGS. 2A and 2B are diagrams showing contact resistance evaluationresults;

FIGS. 3A and 3B are diagrams for explaining a contact resistancereduction effect in the semiconductor device of the first embodiment;

FIG. 4 is a diagram showing a voltage-current characteristic of acontact structure of the semiconductor device of the first embodiment;

FIG. 5 is a diagram showing the relation between NiO film thickness andrise voltage of an LD in the first embodiment;

FIGS. 6A and 6B are schematic cross sections of a semiconductor deviceof a second embodiment; and

FIG. 7 is a diagram showing rise voltages of LDs of example 1 andcomparative example 1.

DETAILED DESCRIPTION

A semiconductor device of an embodiment includes: a semiconductor layermade of p-type nitride semiconductor; an oxide layer formed on thesemiconductor layer, the oxide layer being made of a crystalline nickeloxide, and the oxide layer having a thickness of 3 nm or less; and ametal layer formed on the oxide layer.

Embodiments of the invention will be described below with reference tothe drawings.

First Embodiment

A semiconductor device of the embodiment includes: a p-type nitridesemiconductor layer (a semiconductor layer); a polycrystalline nickeloxide layer (an oxide layer) formed on the p-type nitride semiconductorlayer and having a thickness of 3 nm or less; and a metal layer formedon the nickel oxide layer.

FIGS. 1A and 1B are cross sections of the semiconductor device of thefirst embodiment. The semiconductor device is a laser diode (LD) formedof a GaN (nitride gallium)-based semiconductor. FIG. 1A is a schematiccross section of the entire device, and FIG. 1B is an enlarged crosssection of a contact structure of a p-side electrode.

In the laser diode as the semiconductor device of the embodiment, on thetop face (one face) of a substrate 10 made of an n-type GaN (galliumnitride) semiconductor, as a GaN-based n-type nitride semiconductorlayer 12, for example, an n-type cladding layer of Si-doped n-typeAl_(0.05)Ga_(0.95)N and an n-type guide layer of Si-doped n-type GaN areformed.

On the n-type nitride semiconductor layer 12, an active layer 14 havinga multiple structure of a GaN-based semiconductor having a multiplequantum well structure (MQW), for example,In_(0.12)Ga_(0.88)N/In_(0.03)Ga_(0.97)N is formed. The active layer 14is a light emitting layer which emits light.

On the active layer 14, as a GaN-based p-type nitride semiconductorlayer 16, for example, a p-type guide layer of Mg-doped p-type GaN, ap-type cladding layer of Mg-doped p-type Al_(0.05)Ga_(0.95)N, and ap-type contact layer of p-type GaN doped with about 1×10²⁰ cm⁻³ of Mgare formed.

The p-type nitride semiconductor layer 16 is provided with a ridgestripe 18 for forming a laser beam waveguide region. Side faces of theridge stripe 18 and the surface of a p-type cladding layer 16 b arecovered with, for example, an insulating film 20 of a silicon oxidefilm.

An n-side electrode 24 is provided on the under face (the other face) ofthe substrate 10, and a p-side electrode 26 is provided on the surfaceof the ridge stripe 18 of the p-type nitride semiconductor layer 16.

The details of the p-side electrode 26 forming the p-side contactstructure will be described below with reference to FIG. 1A.

The p-side electrode 26 on p-type GaN in the uppermost part of thep-type nitride semiconductor layer 16 is formed by an NiO (nickel oxide)film 30 as a nickel oxide layer formed on p-type GaN and a metal layer32 formed on the NiO film 30.

The NiO film 30 is made of polycrystal having a thickness of 3 nm orless and in the form of a layer. Desirably, the thickness of the NiOfilm 30 is 0.5 nm or larger from the viewpoint of stably forming apolycrystal film having uniform film thickness.

The metal layer 32 has a layer-stack structure of an Ni (nickel) film 32a which is in contact with the nickel oxide layer, an Au (gold) film 32b, a Ti (titanium) film 32 c, a Pt (platinum) film 32 d, and an Au(gold) film 32 e. The Ni (nickel) film 32 a has the function ofimproving adhesion between the NiO film 30 and the Au (gold) film 32 b.

In the laser diode of the embodiment, between the p-type GaN as thep-type nitride semiconductor layer 16 and the metal layer 32, a contactstructure provided with the NiO film 30 which is very thin, in the formof a layer, and is polycrystal is formed. By the contact structure,contact resistance between the p-type nitride semiconductor layer 16 andthe metal layer 32 is reduced, and rising voltage is reduced. Therefore,the operating voltage of the laser diode can be reduced. Thus,power-light conversion efficiency of the laser diode improves.

Since the NiO film 30 is not amorphous but is crystal, the reliabilityof the laser diode improves. A factor of improvement in reliability isconsidered that since the film is crystal, an oxygen defect in the filmis little, and film property change caused by movement of the oxygendefects in the film during use of the device is suppressed. Anotherfactor of improvement in reliability is as follows. Since the film iscrystal, even in the case where high-density current is passed, thebarrier property to diffusion of a metal improves, and diffusion of themetal to the p-type nitride semiconductor layer is suppressed.

FIGS. 2A and 2B are diagrams showing contact resistance evaluationresults. FIG. 2A shows the case where there is no NiO film between thep-type nitride semiconductor layer and the metal layer, and FIG. 2Bshows the case where there is the NiO film between the p-type nitridesemiconductor layer and the metal layer like in the embodiment.Evaluations are made on assumption that the thickness of the NiO film is1 nm. For the evaluations, the TLM (Transmission Line Model) methodusing a substrate of p-type GaN is employed.

As obvious from FIG. 2, by making the NiO film interpose, the contactresistance is reduced to almost the half.

FIGS. 3A and 3B are diagrams for explaining a contact resistancereduction effect in the semiconductor device of the first embodiment.FIG. 3A is a band diagram showing the case where there is no NiO filmbetween the p-type nitride semiconductor layer and the metal layer, andFIG. 3B is a band diagram showing the case where there is the NiO filmbetween the p-type nitride semiconductor layer and the metal layer likein the embodiment.

As shown in FIG. 3B, by interposition of the NiO film having a bandgapsmaller than that of the p-type nitride semiconductor layer between thep-type nitride semiconductor layer and the metal layer, the barrierbetween the p-type nitride semiconductor layer and the metal layer comesto have two stages Es₁ and Es₂. Both of Es₁ and Es₂ are smaller than abarrier Es₀ in the case where there is no NiO film. It is consideredthat the contact resistance is reduced by the decrease in the barrier.

FIG. 4 is a diagram showing a voltage-current characteristic of acontact structure of the semiconductor device of the first embodiment.It is understood that by providing the NiO film between p-type GaN andthe metal layer, the ohmic characteristic improves. The reason why thenon-ohmic characteristic becomes conspicuous when there is no NiO filmis considered that the barrier between p-type GaN and the metal layer ishigh, and the barrier layer does not become low to the degree thatcarrier injection occurs due to heat release until the electric fieldbecomes large to a certain degree. On the contrary, in the case wherethe NiO film is provided, the barrier between p-type GaN and the metallayer becomes low as described in the above model. Consequently, it isconsidered that carrier injection occurs due to heat release even at lowvoltage.

When the thickness of the crystal NiO film formed in a layer shapeexceeds 3 nm, the contact resistance rises, and the rise voltage of thelaser diode increases. FIG. 5 is a diagram showing the relation betweenNiO film thickness and rise voltage of the LD. The rise voltage of thelaser diode shown in FIG. 1 is obtained by simulation.

Since the light emission wavelength of the laser diode of the embodimentis around 400 nm, ideally, 3 V corresponding to the energy of thebandgap is the rise voltage. As shown in FIG. 5, when the thickness ofthe NiO film is up to 3 nm, the rise voltage is almost 3 V. However,when the thickness exceeds 3 nm, the rise voltage rises.

It is generally considered that electric conduction in NiO is carriedout by an oxygen defect caused by the amorphous state or insufficientoxidation. Consequently, in the case of the amorphous state orinsufficient oxidation, even when the film thickness increases,resistance which is enough for practical use is obtained.

In the embodiment, the film is crystal and an oxygen defect is small, sothat the resistance of the NiO film itself becomes high. In thesimulation, perfect crystal is assumed, and conduction due to an oxygendefect is not considered. Consequently, the resistance component of theNiO film itself becomes large.

In the region of 3 nm or less, the film becomes thin, and tunnelinjection of carries becomes possible. As a result, the rise voltagesharply drops. In addition, as described above, the barrier betweenp-type GaN and the metal layer becomes lower, so that almost ideal risevoltage of the LD is obtained.

In the embodiment, desirably, crystal grain size of the nickel oxidelayer is larger than thickness of the nickel oxide layer. In thestructure, the grain boundary does not exist or hardly exists in thethickness direction of the nickel oxide layer. Therefore, thereliability of the nickel oxide layer improves, and the reliability ofthe laser diode also improves.

The crystal grain size of the nickel oxide layer is obtained by, forexample, measuring a plurality of greatest sizes of crystal grainsobserved in a TEM photograph or the like and averaging them.

Next, a method of manufacturing the semiconductor device of theembodiment will be described.

The substrate 10 of n-type GaN (gallium nitride) semiconductor in thewafer shape is subjected, for example, to pretreatment using an organicsolvent and acid. After that, by known MOCVD, while switching materialgases, the n-type nitride semiconductor layer 12, the active layer 14 ofthe multiple structure, and the p-type nitride semiconductor layer 16are formed.

Next, the ridge stripe 18 is formed by known dry etching. The insulatingfilm 20 as a silicon oxide film is formed. After that, the insulatingfilm 20 on the ridge stripe 18 is removed.

Next, by electrode deposition in a vacuum apparatus, an Ni film having athickness of 3 nm or less is deposited on the p-type nitridesemiconductor layer 16. After that, heat treatment of 450° C. or less isperformed at normal pressure in oxygen-nitrogen mixed atmosphere tooxidize the Ni film and to form the NiO film 30 of polycrystal as anickel oxide layer.

Desirably, the oxygen-nitrogen mixed atmosphere is atmosphere havingoxygen of 20% or less. From the viewpoint of increasing the grain sizeof the crystal, more desirably, the heat treatment temperature is 400°C. or less.

Next, by electrode deposition in a vacuum apparatus, on the NiO film 30,the Ni (nickel) film 32 a, the Au (gold) film 32 b, the Ti (titanium)film 32 c, the Pt (platinum) film 32 d, and the Au (gold) film 32 e aredeposited to form the p-side electrode 26. After that, for example, heattreatment may be performed for annealing in the nitrogen atmosphere.

Subsequently, the side opposite to the p-side electrode 26 of thesubstrate 10 is thinned by polishing. A Ti (titanium) film, a Pt(platinum) film, and an Au (gold) film are deposited to form the n-sideelectrode 24.

The wafer is cut by cleavage to form a resonator mirror. After that, thewafer is separated into chips. One side of the resonator mirror isformed by, for example, a high-reflection film as a dielectricmultilayer film, and a light emission face is formed by a low-reflectionfilm. After that, the chip is mounted on a heat sink.

By the above manufacturing method, the semiconductor device of theembodiment is manufactured.

In the manufacturing method, by performing the heat treatment of 450° C.or less on the Ni film in the oxygen-nitrogen mixed atmosphere,sufficient oxidation is realized stably, and the very-thin polycrystalNiO film having a layer shape and large crystal grain size can bemanufactured.

Second Embodiment

A second embodiment is different from the first embodiment with respectto the point that the semiconductor device is not the laser diode (LD)but is a light emitting diode (LED). The second embodiment is differentfrom the first embodiment with respect to the point that the metal layeron the NiO film has a layer-stack structure of an Ag (silver) film, a Ti(titanium) film, a Pt (platinum) film, and an Au (gold) film. The actionand effect produced by providing the NiO film are similar to those ofthe first embodiment. Therefore, the contents overlapping with the firstembodiment will not be repeated.

FIGS. 6A and 6B are schematic cross sections of the semiconductor deviceof the second embodiment. The semiconductor device is a light emittingdiode (LED) formed of a GaN (nitride gallium)-based semiconductor. FIG.6A is a schematic cross section of the entire device, and FIG. 6B is anenlarged cross section of a contact structure of a p-side electrode.

In the light emitting diode as the semiconductor device of theembodiment, for example, on a light transmissive substrate 40 ofsapphire, a guide layer made of Si-doped n-type GaN is formed as aGaN-based n-type nitride semiconductor layer 42. Between the lighttransmissive substrate 40 and the n-type nitride semiconductor layer 42,for example, a buffer layer of AlN may be provided. The lighttransmissive substrate 40 is a substrate which transmits light emittedby the active layer, for example, visible light.

On the n-type nitride semiconductor layer 42, an active layer (lightemitting layer) 44 which emits light is formed. The active layer 44, forexample, has the MQW (multiple quantum) formed of InGaN-based nitridesemiconductor. The layer stack structure of a barrier layer, a quantumwell layer, and a barrier layer is repeated, for example, a plurality oftimes.

On the active layer 44, as a GaN-based p-type nitride semiconductorlayer 46, for example, a p-type guide layer of Mg-doped P-type GaN, anda p-type contact layer of p-type GaN doped with about 1×10²⁰ cm⁻³ of Mgare formed.

An n-side electrode 54 is provided in a region in which the active layer44 is not formed, in the n-type nitride semiconductor layer 42. Then-side electrode 54 has, for example, a layer stack structure of a Tifilm, a Pt film, and an Au film.

Further, a p-side electrode 56 is provided on the p-type nitridesemiconductor layer 46. The p-side electrode 56 functions also as areflection electrode which reflects light emitted from the active layer44 to the light transmissive substrate 40 side. The details of thep-side electrode 56 forming the p-side contact structure will bedescribed below with reference to FIG. 6B.

The p-side electrode 56 on p-type GaN in the uppermost part of thep-type nitride semiconductor layer 46 is formed by the NiO (nickeloxide) film 30 as a nickel oxide layer formed on p-type GaN and a metallayer 62 formed on the NiO film 30.

The NiO film 30 is made of polycrystal having a thickness of 3 nm orless and in the form of a layer. Desirably, the thickness of the NiOfilm 30 is 0.5 nm or larger from the viewpoint of forming a polycrystalfilm having uniform film thickness.

The metal layer 62 has a layer-stack structure of an Ag (silver) film 62a which is in contact with the nickel oxide layer, a Ti (titanium) film62 b, a Pt (platinum) film 62 c, and an Au (gold) film 62 d.

In the light emitting diode of the embodiment, between the p-type GaN asthe p-type nitride semiconductor layer 46 and the metal layer 62, acontact structure provided with the NiO film 30 which is very thin, inthe form of a layer, and is polycrystal is formed. By the contactstructure, contact resistance between the p-type nitride semiconductorlayer 46 and the metal layer 62 is reduced, and the operating voltagecan be therefore reduced. Thus, power-light conversion efficiency of thelight emitting diode improves.

Since the NiO film 30 is not amorphous but is crystal, the reliabilityof the light emitting diode improves. A factor of improvement inreliability is considered that since the film is crystal, an oxygendefect in the film is little, and film property change caused bymovement of the oxygen defects in the film during use of the device issuppressed. Another factor of improvement in reliability is consideredas follows. Since the film is crystal, even in the case wherehigh-density current is passed, the barrier property to diffusion of ametal improves, and diffusion of the metal to the p-type nitridesemiconductor layer is suppressed.

By providing the NiO film 30 which is very thin, formed in the form of alayer, and is polycrystal between the p-type GaN as the p-type nitridesemiconductor layer 46 and the metal layer 62 serving as a reflectionelectrode, the reflectance of the reflection electrode improves.

Next, a method of manufacturing the semiconductor device of theembodiment will be described.

On the light transmissive substrate 40 of sapphire having a wafer shape,for example, using AlN as a buffer layer, an n-type GaN as the n-typenitride semiconductor layer 42, the active layer 44 of an InGaN-basednitride semiconductor, and p-type GaN as the p-type nitridesemiconductor layer 46 are formed by epitaxial growth by the knownMOCVD.

Next, for example, the wafer is processed with aqua regia and an Ni filmhaving a thickness of 3 nm or less is deposited on the p-type nitridesemiconductor layer 46 by electrode deposition in a vacuum apparatus.Further, by electrode deposition in a vacuum apparatus, on the Ni film,the Ag (silver) film 62 a, the Ti (titanium) film 62 b, the Pt(platinum) film 62 c, and the Au (gold) film 62 d are deposited to formthe p-side electrode 56.

After that, for example, heat treatment of 450° C. or less is performedat normal pressure in oxygen-nitrogen mixed atmosphere to oxidize the Nifilm and to form the NiO film 30 of polycrystal as a nickel oxide layer.When the temperature exceeds 450° C., it is unpreferable since there isthe possibility that the NiO film is formed in an island shape, not alayer shape.

Desirably, the oxygen-nitrogen mixed atmosphere is atmosphere havingoxygen of 20% or less. From the viewpoint of increasing the grain sizeof the crystal, more desirably, the heat treatment temperature is 400°C. or less.

Next, patterning is performed with a resist, and the n-type nitridesemiconductor layer 42 in the region where the n-side electrode 54 isformed is exposed by dry etching. Next, patterning is performed so thatthe region except for the region where the n-side electrode 54 is to beformed is covered with the resist.

Next, by electrode deposition in a vacuum apparatus, a Ti (titanium)film, a Pt (platinum) film, and an Au (gold) film are deposited onn-type GaN, and the n-side electrode 54 is formed by the lift-offmethod. To protect a region in which an electrode is not formed, aprotection film of an insulating film may be properly formed in themanufacturing process.

After that, the wafer is diced into chips. The chip is mounted on a heatsink.

By the above manufacturing method, the semiconductor device of theembodiment is manufactured.

In the manufacturing method, by performing the heat treatment of 450° C.or less on the Ni film in the oxygen-nitrogen mixed atmosphere,sufficient oxidation is realized, and the very-thin polycrystal NiO filmhaving a layer shape and large crystal grain size can be manufactured.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the semiconductor device describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the devices andmethods described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

For example, with respect to the structure of the metal layer, thestructure other than the layer stack structures described in theembodiments can be applied. For example, an Al film can be also appliedin place of an Au film or an Ag film on the NiO film.

Any layer configurations, film thicknesses, compositions, and the likeof the nitride semiconductor layer and the active layer of the laserdiode and the light emitting diode may be employed as long as theyrealize the functions of the semiconductor light emitting device. Thepresent invention is not limited to the configurations of theembodiments.

Although the semiconductor light emitting devices such as the laserdiode and the light emitting diode have been described as examples inthe embodiments, for example, the invention can be also applied to thecontact structure in any of the other semiconductor devices such as ahigh-output heterojunction bipolar transistor using nitridesemiconductor for a substrate.

Although the GaN-based semiconductor has been described as an example ofthe nitride semiconductor, the invention can be also applied to theother nitride semiconductors such as AlN-based and InN-based nitridesemiconductors.

EXAMPLES

Examples will be described below.

Example 1

A semiconductor laser diode similar to that described in the firstembodiment was formed.

On a wafer-shaped n-type GaN (gallium nitride) semiconductor substrate,as a GaN-based n-type nitride semiconductor layer, an n-type claddinglayer of Si-doped n-type Al_(0.05)Ga_(0.95)N and an n-type guide layerof Si-doped n-type GaN were formed.

On the n-type nitride semiconductor layer, an active layer having amultiple structure of In_(0.12)Ga_(0.88)N/In_(0.03)Ga_(0.97)N, of aGaN-based semiconductor having a multiple quantum well structure (MQW)is formed.

On the active layer, as a GaN-based p-type nitride semiconductor layer,an undoped GaN guide layer, a p-type overflow preventing layer ofMg-doped p-type Al_(0.2)Ga_(0.8)N, a p-type cladding layer of Mg-dopedp-type Al_(0.05)Ga_(0.95)N, and a p-type contact layer of p-type GaNdoped with about 1×10²⁰ cm⁻³ of Mg were formed.

A ridge stripe was formed by known dry etching. An insulating film as asilicon oxide film was formed. After that, the insulating film on theridge stripe was removed by acid treatment.

Next, by electrode deposition in a vacuum apparatus, an Ni film having athickness of 1 nm was deposited on p-type GaN as a p-type contact layer.After that, heat treatment of 395° C. was performed for one minute atnormal pressure in oxygen-nitrogen mixed atmosphere containing 20% ofoxygen to oxidize the Ni film and to form a NiO film.

Next, by electrode deposition in a vacuum apparatus, on the NiO film, anNi film having a thickness of 2 nm, an Au film having a thickness of 100nm, a Ti film having a thickness of 100 nm, a Pt film having a thicknessof 50 nm, and an Au film having a thickness of 500 nm were deposited toform a p-side electrode.

Subsequently, the side opposite to the p-side electrode 26 of thesubstrate 10 was polished so that the wafer is thinned to a thickness of150 μm. A Ti film having a thickness of 100 nm, a Pt film having athickness of 50 nm, and an Au film having a thickness of 500 nm weredeposited to form an n-side electrode.

The wafer was cut by cleavage to forma resonator mirror. After that, thewafer was separated into chips. One side of the resonator mirror wasformed by a high-reflection film as a dielectric multilayer film, and alight emission face was formed by a low-reflection film. After that, thechip was mounted on a heat sink.

The operating voltage of the device was 4.8 V in 2W operation. The risevoltage was 3.0 V.

In observation of the p-side electrode part with a transmission electronmicroscope (TEM), the NiO film was formed in a layer shape having athickness of 1 nm almost uniformly and was polycrystal. The crystalgrain size of the NiO film was larger than the thickness of the NiO filmand, a so-called bamboo structure was formed.

When the ratio between the number of atoms of Ni (nickel) of the NiOfilm and the number of atoms of O (oxygen) was checked by photoelectronspectrometry (XPS) immediately after oxidation in the heat treatment, itwas Ni:O=1:0.99. Also in an analysis in a time-of-flight secondary ionmass spectrometry apparatus (TOF-SIMS), Ni:O=1:0.98. In such a manner,it was confirmed that a NiO crystal having a composition extremely closeto a stoichiometric composition is formed.

The reliability of the device was evaluated. An acceleration test oftemperature of 80° C. was conducted to monitor deterioration in theluminous efficiency. By a test of 10,000 hours, deterioration of 10% wasobserved.

Comparative Example 1

A laser diode was manufactured by a manufacturing method similar to thatof Example 1 except that the thickness of the Ni film was set to 5 nm.

The rise voltage was 4.5 V. In a manner similar to Example 1, byobservation with a transmission electron microscope (TEM), it wasrecognized that the NiO film was formed in a layer shape having athickness of 5 nm almost uniformly and was polycrystal.

FIG. 7 is a diagram showing rise voltages of LDs of example 1 andcomparative example 1. A simulation result shown in FIG. 5 will be alsodescribed. Consistency between simulation and actual measurement on therelation between the NiO film thickness and the rise voltage wasrecognized.

Comparative Example 2

A laser diode was manufactured by a manufacturing method similar to thatof Example 1 except that heat treatment after formation of an Ni filmwas not performed, and after deposition, on an NiO film, of an Au filmhaving a thickness of 100 nm, a Ti film having a thickness of 100 nm, aPt film having a thickness of 50 nm, and an Au film having a thicknessof 500 nm, heat treatment of 550° C. for 10 minutes was performed atnormal pressure in oxygen-nitrogen mixed atmosphere having 20% of oxygento oxidize the Ni film.

By observation with a transmission electron microscope (TEM), it wasunderstood that Au was moved, an Au film was formed on a p-type GaNlayer, an NiO film was formed on the Au film, and a Ti film, a Pt film,and an Au film were formed on the NiO film. That is, the NiO film doesnot have a contact structure interposing between p-type GaN and themetal layer.

The reliability of the device was evaluated in a manner similar toExample 1. By a test of 1,000 hours, deterioration of 30% was observed.

Example 2

A semiconductor light emitting diode similar to that described in thesecond embodiment was formed.

On a light transmissive substrate of sapphire having a wafer shape, abuffer layer of AlN, n-type GaN as an n-type nitride semiconductorlayer, an active layer of InGaN, a p-type AlGaN layer as a p-typenitride semiconductor layer, and p-type GaN were formed by epitaxialgrowth in MOCVD. In an uppermost part of p-type GaN, as a p-type contactlayer, p-type GaN doped with about 1×10²⁰ cm⁻³ of Mg was formed.

Next, the wafer was processed with aqua regia and an Ni film having athickness of 2.8 nm was deposited on the p-type GaN as the p-typecontact layer by electrode deposition in a vacuum apparatus. Further, byelectrode deposition in a vacuum apparatus, on the Ni film, an Ag film,a Ti film, a Pt film, and Au film were deposited.

After that, heat treatment of 395° C. was performed for one minute atnormal pressure in oxygen-nitrogen mixed atmosphere containing 20% ofoxygen to oxidize the Ni film and to form a NiO film, thereby forming ap-side electrode.

Next, patterning is performed with a resist, and n-type GaN in theregion where an n-side electrode is to be formed is exposed by dryetching. Next, patterning was performed so that the region except forthe region where the n-side electrode is to be formed is covered withthe resist.

Next, by electrode deposition in a vacuum apparatus, a Ti film, a Ptfilm, and an Au film were deposited on n-type GaN, and an n-sideelectrode was formed by the lift-off method. To protect a region inwhich an electrode is not formed, a protection film of an insulatingfilm was properly formed in the manufacturing process.

The wafer was diced into chips in 300 μm in height and width. Afterthat, the chip was mounted on a heat sink with an Ag paste.

The device expressed 2.8 V in voltage when current was 20 mA. Also inthe case where current is set to 200 mA, the device operates stably, andthe operation voltage was 3.1 V.

In observation of the p-side electrode part with a transmission electronmicroscope (TEM), the NiO film was formed in a layer shape having athickness of 2.8 nm almost uniformly and was polycrystal. The crystalgrain size of the NiO film was larger than the thickness of the NiO filmand, a so-called bamboo structure was formed.

The reliability of the device was evaluated. An acceleration test withconstant current of 1 A was conducted to monitor occurrence of leakcurrent. No leak current occurred also in a test for 5,000 hours, sothat excellent reliability was recognized.

Example 3

A semiconductor light emitting diode was formed by a manufacturingmethod similar to that described in the second embodiment except that anAl (aluminum) film was formed in place of the Ag film in the reflectionelectrode.

The reliability of the device was evaluated in a manner similar toExample 2. No leak current occurred also in a test for 10,000 hours, andexcellent reliability was recognized.

Comparative Example 3

A semiconductor light emitting diode was formed by a manufacturingmethod similar to that described in the second embodiment except that aNb film is not formed between p-type GaN and the Ag film.

The reliability of the device was evaluated in a manner similar toExample 2. A leak current occurred in a test for 100 hours.

What is claimed is:
 1. A semiconductor device comprising: a p-typenitride semiconductor layer; a metal layer; and an oxide layer formedbetween the p-type nitride semiconductor layer and the metal layer, theoxide layer being made of a crystalline nickel oxide, and the oxidelayer having a thickness of 3 nm or less.
 2. The device according toclaim 1, wherein crystal grain size of the oxide layer is larger thanthickness of the oxide layer.
 3. The device according to claim 1,wherein the metal layer includes a silver (Ag) film being in contactwith the oxide layer.
 4. The device according to claim 1, wherein themetal layer includes a nickel (Ni) film being in contact with the oxidelayer, and a gold (Au) film formed on the nickel film.
 5. The deviceaccording to claim 1, wherein the p-type nitride semiconductor layer ismade of p-type gallium nitride (GaN).
 6. A semiconductor devicecomprising: an n-side electrode; a p-side electrode; an n-type nitridesemiconductor layer formed between the n-side electrode and the p-sideelectrode; a p-type nitride semiconductor layer formed between then-type nitride semiconductor layer and the p-side electrode; and anactive layer formed between the n-type nitride semiconductor layer andthe p-type nitride semiconductor layer, the active layer emitting light,wherein the p-side electrode includes a crystalline nickel oxide layerhaving a thickness of 3 nm or less and a metal layer being in contactwith the nickel oxide layer, the p-side electrode reflects light emittedfrom the active layer.
 7. The device according to claim 6, whereincrystal grain size of the nickel oxide layer is larger than thickness ofthe nickel oxide layer.
 8. The device according to claim 6, wherein themetal layer has a silver (Ag) film being in contact with the nickeloxide layer.
 9. The device according to claim 6, wherein the metal layerhas a layer-stack structure of a silver (Ag) film being in contact withthe nickel oxide layer, a titanium (Ti) film, a platinum (Pt) film, anda gold (Au) film.
 10. The device according to claim 6, wherein thep-type nitride semiconductor layer is made of p-type gallium nitride(GaN).
 11. A semiconductor device comprising: a semiconductor substrate;an n-type nitride semiconductor layer formed on one face of thesubstrate; an active layer formed on the n-type nitride semiconductorlayer, the active layer emitting light; a p-type nitride semiconductorlayer formed on the active layer, the p-type nitride semiconductorhaving a ridge stripe; an n-side electrode formed on the other face ofthe substrate; and a p-side electrode formed on the ridge stripe, thep-side electrode including a crystalline nickel oxide layer having athickness of 3 nm or less and a metal layer formed on the nickel oxidelayer.
 12. The device according to claim 11, wherein crystal grain sizeof the nickel oxide layer is larger than thickness of the nickel oxidelayer.
 13. The device according to claim 11, wherein the metal layer hasa nickel (Ni) film being in contact with the nickel oxide layer, and agold (Au) film formed on the nickel film.
 14. The device according toclaim 11, wherein the metal layer has a layer-stack structure of an Ni(nickel) film being in contact with a nickel oxide layer, an Au (gold)film, a Ti (titanium) film, a Pt (platinum) film, and an Au (gold) film.15. The device according to claim 11, wherein the p-type nitridesemiconductor layer is made of p-type gallium nitride (GaN).